Certain substances, when bound to the membranes of neurons, cardiac and skeletal muscle, glands, and other cells, behave as molecular indicators of membrane potential. The optical properties of these molecules vary linearly with potential and may, therefore, be used to monitor action potentials, synaptic potentials, or other changes in membrane voltage from a large number of sites at once, without the use of electrodes. The project will develop more sensitive probes, add to our understanding of the mechanisms by which they transduce voltage, and extend the technology associated with their use. The intention is to use these molecular voltmeters for optical recording of membrane potential from otherwise inaccessible regions of single neurons such as nerve terminals and dendritic arborizations, and from many sites simultaneously in small ensembles of neurons in tissue culture in order to study the spatial and temporal patterning of activity. First, a new high resolution computer based system for Multiple Site Optical Recording of Transmembrane Voltage (MSORTV) will be constructed, capable of monitoring changes in membrane potential from as many as 464 at once, and this apparatus will be used to study the properties of truly simple nervous systems - small ensembles of synaptically connected Aplysia central neurons maintained in culture - by optical recording of electrical activity from all of their components simultaneously. Second, this apparatus will be used to record membrane potential changes from fine processes of single neurons in order to determine the electrical properties, both active and passive, of neuronal structures which are not penetrable by electrodes and are frequently too far away, electrically, for their activity to be reflected in the somata. Finally, the project will exploit the laboratory's unique ability to monitor electrical activity in the nerve terminals of vertebrates to study the ion channels at the release sites of neuropeptides, and to combine absorption measurements of excitation at nerve terminals with photon correlation studies, using laser light scattering, in order better to understand how excitation is coupled to secretion, in vertebrate neurons and elsewhere.